vidcontrol: disable p, P, and H when vt(4) is in use

These options use the CONS_SCRSHOT ioctl to capture the contents of the
current console, which is not yet supported by vt(4). Disable the
options when vt(4) is in use rather than emitting a possibly confusing
error message.

This change should be reverted if CONS_SCRSHOT is implemented for vt(4).

PR: 263099
MFC after: 1 week
Sponsored by: The FreeBSD Foundation